Shredding mills are used for salvage purposes in converting junked automobiles into fragments that may be used in recycling processes. Automobile shredding mills typically comprise apparatus that rotates relative to a cutter bar at the end of a feeder chute through which the junked automobile is fed. The rotating apparatus conventionally comprises four or six sets of hammers, each set including a plurality of hammers mounted for swinging movement on a common pivot shaft. The pivot shafts are carried between large structural spiders that are mounted on the large central drive shaft. The pivot shafts are equiangularly disposed around the drive shaft, which typically rotates at 720 revolutions per minute. For each revolution, each of the four or six rows of hammers passes by the cutter bar, shearing or shredding the automobile as it is fed along the entry chute.
The rotating sets of hammers are enclosed in a housing that is generally cylindrical in configuration, comprising in part a number of solid liner sections, and also comprising a plurality of discharge grates that are radially oriented and spaced apart to define discharge openings. The fragments resulting from the shredding process are dropped from the grates onto a conveyor.
One of the problems associated with automobile shredding mills is replacement of the hammers, which wear down during the shredding process and require replacement on the order of every twenty days. The individual hammers are relatively heavy, weighing from 225-480 pounds, necessitating the use of lifting equipment in the removal and installation process. The problem is compounded by the fact that the hammers of each set are commonly mounted on a single pivot shaft, requiring all of the hammers to be suspended simultaneously for both removal and installation.
This problem has been dealt with by including a small lifting eye on each hammer suitable for lifting the hammer through the use of a grappling hook or the like. The lifting eye works adequately during installation of new hammers since it is intact at the time of fabrication. However, because the lifting eyes form part of the hammers themselves, they are broken by impact, or worn away quite easily during the shredding process, and as a result there is no structure by which the hammers can be lifted during replacement.
A related problem that leads to the same disadvantageous result is the difficulty in fabricating lifting eyes on hammers. It is quite difficult to cast members such as conventional lifting eyes from hardened alloys without encountering cracks of some type. Where such cracks occur and are immediately perceived during the casting process, the entire hammer must be scrapped, resulting in wasted material and time. If the cracks are not immediately perceived, it is possible for the lifting eye to fragment during the shredding process, again leaving the hammer without any structural means for lifting it out during replacement.
The problems of lifting eye wear and fragmenting become more critical in those shredding mills which use opposed long rolls and short rolls in the rotating apparatus. A "long roll" refers to a set of new hammers having full length. The term "short roll" refers to a set of hammers which have been used for a period of time and have a shorter length by reason of wear.
In mills using both long rolls and short rolls, the partially worn hammers are advantageously used, but they must be installed, reversed, and removed at least twice during their useful life. If the lifting eye wears or fragments at any time prior to the second removal, the hammer cannot be removed with conventional means.
The inability to remove and install or reinstall hammers because of the absence of lifting eyes adds a significant amount of time to the replacement process and increases the downtime of the mill. A typical solution has been to weld new lifting eyes onto old hammers simply to permit their removal. Thus, the problem and its existing solution requires increased downtime, increased expenditure of materials and labor, and a decrease in productivity of the mill. Welding on a hardened alloy increases the risk of hammer failure by cracking. At full speed, such failures are extremely hazardous.